As an image taking method related to magnetic resonance imaging apparatuses, a method called “Fast Spin Echo (FSE) method” is conventionally known. The FSE method is an image taking method by which a plurality of echo signals called an echo train are collected by applying a flip pulse to an examined subject and then sequentially applying a plurality of flop pulses to the examined subject. In this situation, the flip pulse is a Radio Frequency (RF) pulse used for exiting atomic nuclear spins within the examined subject. The flop pulses are RF pulses used for refocusing phases of the atomic nuclear spins.
According to the FSE method described above, because the plurality of RF pulses are applied, a stimulated echo is generated together with a spin echo. In some situations, the stimulated echo causes a phase shift in the collected echo signals. Such phase differences among the echo signals may be a cause of degradations of image quality such as sensitivity unevenness, lowering of signal strength, “ghost” phenomena, and the like.
To prevent such degradations of the image quality, generally speaking, a pre-scan for measuring the phase differences among the echo signals is performed prior to a main scan, so that a pulse sequence for the main scan can be corrected based on the phase differences measured during the pre-scan. In this situation, for example, a pulse sequence that cancels the stimulated echo is executed during the pre-scan so that only the spin echo is collected. Further, of the spin echo signals collected during the pre-scan, the first and the second echo signals are Fourier-transformed in a readout direction, so that a zero-order phase difference and a first-order phase difference between the first echo signal and the second echo signal can be calculated. After that, a correction amount is calculated so as to correct a readout direction phase shift and a slice selecting direction phase shift, based on the zero-order phase difference and the first-order phase difference being calculated. The pulse sequence for the main scan is changed, based on the calculated correction amount.
However, according to the conventional technique described above, it is difficult to correct the phase difference occurring in the phase encoding direction by the influence of an overcurrent caused by the phase encoding gradient magnetic field and image quality is likely to deteriorate due to the phase difference.